The present invention relates to a process for the synthesis of methyl mercaptan from hydrogen sulfide and methanol.
Although methyl mercaptan can be produced catalytically from various starting materials, such as carbon disulfide, carbon monoxide or methane, the most important method from a commercial standpoint consists of reacting hydrogen sulfide and methanol in the vapor phase in the presence of a catalyst essentially as described by Sabatier in 1910 (P. Sabatier, and A. Mailhe, compterendu 150, 832-6, 1569-72, 1217-21, 1910). Since that time, it has been possible to gradually increase the degree of methanol conversion above the 50% level obtained by Sabatier et al by utilizing more effective catalysts. Today, due especially to the use of catalysts based on activated alumina and promoters such as potassium oxides or salts, the problem posed by the synthesis of methyl mercaptan is no longer so much that of the degree of conversion of methanol as that of the selectivity of the reaction for the desired product. In the course of the reaction, a number of objectionable by-products are formed, the primary one being dimethyl sulfide.
It has previously been recognized that the degree of methanol conversion is increased by increasing the temperature of the reaction, while the selectivity in favor of methyl mercaptan is decreased by elevated temperature conditions. Moreover, a temperature threshold exists below which the reaction is not initiated when employing the usual catalysts based on alumina. In practice, therefore, the reactants are introduced into the catalyst mixture at a temperature above 280.degree.C. Since the reaction is exothermic, the reaction temperature rapidly escalates, and this, as stated above, favorably affects the degree of conversion of the methanol but adversely affects the selectivity of the reaction. As disclosed in French Pat. No. 1,161,066, filed June 22, 1956, maintaining the reaction mixture at a uniform temperature very selectively promotes the formation of methyl mercaptan and, consequently, minimizes undesired by-products such as dimethyl sulfide. It can readily be seen then that controlling the temperature is a significant consideration in carrying out the synthesis of methyl mercaptan.
The temperature control method proposed in the abovementioned French patent involves a combination of external and internal temperature regulative means. Externally, the reaction mixture is cooled by any means which makes it possible to remove heat from the reaction mixture, for example, by using a tubular reactor through which a heat exchange fluid, such as air or an organic liquid with a high boiling point, flows. Internal temperature regulation is achieved by utilizing a high molar ratio of hydrogen sulfide to methanol (greater than 2.5) making it possible to use the excess hydrogen sulfide as a heat absorbing agent, and moreover, promoting the formation of methyl mercaptan due to the increased selectivity of the reaction observed with a high H.sub.2 S/CH.sub.3 OH molar ratio.
While the foregoing solution has proved to be valuable, there are a number of disadvantages associated therewith. One such disadvantage is the necessity of using means external to the reaction to control the temperature instead of confining control to the regulation of reaction parameters. The distinctive drawback of utilizing external control is that it makes it extremely difficult to rapidly lower the reaction temperature in the event of serious exothermic conditions. Moreover, as a consequence of the elevated temperature prevailing in the reaction system and the fluctuation thereof occasioned by unstable exothermic conditions, the selection of the proper heat exchange fluid is necessarily somewhat arbitrary which gives rise to unpredictable temperature control often resulting in serious deterioration and corrosion within the reactor. It should also be noted that even where optimum heat exchange conditions exist, a severe hot spot is observed at the inlet to the reactor causing premature destruction of the catalyst as well as the increased production of methanol cracking products. While the deleterious affect on the catalyst can be partially avoided by diluting the catalyst in the inlet zone, this presents difficult charging problems and problems related to maintaining the homogeneity of the catalyst charge in the reactor.
The utilization of a high H.sub.2 S/CH.sub.3 OH molar ratio represents a serious disadvantage in the sense that it necessitates making the reactor unit larger to accommodate the excess feed, resulting in higher investment and operating costs. Moreover, a large excess of hydrogen sulfide leads to a reduction in the amount of methyl mercaptan recovered as a result of the entrainment of the product in the uncondensed gases removed after the reaction.